1. Field
This invention relates to reflection mode ultrasonic imaging systems. It is particularly directed to a synthetic focusing approach useful for enhancing the images obtainable from an annular array scanner.
2. State of the Art
The B-scan is a pulse echo technique where a pulse is sent into the object to be imaged and the echoes from the object are received by the same transducer used for transmitting. The positions of the echoes in time along with transducer positioning information allow the reconstruction of tissue interface or reflector positions. The imaging target is usually assumed to have a constant speed of sound throughout the object. It is also usually assumed that sound propagates in a straight line. Spatial position s(t), may then be determined as a function of time: ##EQU1## where c.sub.0 is the average speed of sound of the object. Since c.sub.0 is an approximation, there will be errors in position and the reconstruction associated with this approximation. It is known that biological soft tissue has about a .sup.+/- 5% variation. The approximation that the velocity of sound is the same as water throughout an imaging target consisting of biological tissue will cause some improper mapping of time signal to spatial position.
Arrays of transducers have allowed the usage of various phased array or beam-steering techniques. This technique involves delaying signals from the different transducer elements in the array. The signals are delayed in a manner to achieve a dynamic focusing effect. These delayed signals are summed in a fashion to pick out only those signals representing the same phase front. This technique makes use of approximate transducer position in order to reconstruct the peak reflection locations reflected from the object to be imaged. Variations in the speed of the sound in an object to be imaged will provide additional variations in the required delays. Failure to account for these approximations also cause some improper mapping of time signal to spatial position.
The single view synthetic focusing algorithm also can make use of the phased array technique. The first synthetic focusing reconstructions were also based on the straight line path approximation as are most of the beam-steering methods developed to date. Current known imaging methods all make use of approximations of constant speed of sound, straight line path of sound propagation, and in transducer location.
A more advanced implementation of the synthetic focusing algorithm makes use of many views of the object to be imaged and combines them into one image with improved resolution. This implementation makes use of approximations in the view angle rotation and the relative center of the rotation of the object to be imaged. Inaccurate approximations in the view angle rotation and relative center of rotation increase the reconstruction blurring and reduces the resolution of the image.
In general imaging systems, a distortion occurs because of the inherent processes used in reconstructing the image. The distorting function is commonly known in the imaging field as the point spread function. Designers of various imaging systems can make theoretical approximations of this point spread function in order to minimize its effect. Calculations for more complicated imaging systems require more crude approximations because the complete characterization of the causes of these distortions may not be possible. This leads to poorer resolution in the reconstructions from imaging systems that attempt the removal of the point spread functions by theoretical approximations.
Among the publications regarded as informative concerning the state of the art as it pertains to the present invention are: S. J. Norton and M. Linzer, "Correction for ray refraction in velocity and attenuation tomography: a perturbation approach," Ultrasonic Imaging 4, pp. 201-233, New York, N.Y.: Plenum Press, 1982; C. H. Knapp and G. C. Carter, "The generalized correlation method for estimation of time delay," IEEE Transactions on Acoustics, Speech, and Signal Processing, vol. ASSP-24, no. 4, pp. 320-327, August 1976; D. Hiller and H. Ermert, "System analysis of ultrasound reflection mode computerized tomography," IEEE Transactions on Sonics and Ultrasonics, vol. SU-31, no. 4, pp. 240-250, July 1984; F. J. Millero, "Speed of sound in seawater as a function of temperature and salinity at 1 atm," J. Acoust. Soc. Am., vol. 57, no. 2, pp. 312-319, February 1975; and R. C. Weast ed., CRC Handbook of Chemistry and Physics, Boca Raton, Fla.: CRC Press, 1981.